1. Field of the Invention
The invention relates to a process of manufacturing soft polyurethane foams on the basis of polyethers. Such foams have recently attracted increasing attention as "cold foams" or "high resilience foams (HR foams)".
2. Description of the Prior Art
There are many process variations for producing HR foams which are extensively described in the chemical literature. Polyethers, crosslinkers and polyisocyanates may be varied as much as the auxiliaries used, such as catalysts, emulsifiers, propellants, etc. Common to these processes is the fact that, as the result of early crosslinking of the polyurethane foam, no polysiloxane-polyether copolymers need be used as stabilizing additive to prevent collapse of the foam. As a matter of fact, the use of the indicated stabilizing additives must oftentimes be avoided since the foam stabilizers -- which are essential in the manufacture of "hot foams" that have hitherto dominated the field -- lead in "cold foams" to an intolerable shrinkage tendency and frequently, to an equally undesirable uniformity of the pore structure of the foam.
Many papers have been published about the state of the art, representative examples of which are: German Offenlegungsschrifts Nos. 2,221,811, 2,246,400, 2,337,140 and 2,356,443, as well as various articles published in the Journal of Cellular Plastics, such as January/February issue 1972, pages 35 to 44; January/February issue 1973; pages 19 to 24; March/April issue 1973, pages 92 to 98 and May/June issue 1973, pages 134 to 138.
The invention process is primarily concerned with foam materials which, in principle, can be manufactured without the addition of any organosiloxane. The skilled art worker is aware that such foam materials can be produced by achieving a relatively high incidence of crosslinking in the polyurethane structure, up to the time of "opening" of the foam material. From practical experience and from the literature, different methods are known for achieving this degree of crosslinking: for example, through the use of a polyisocyanate with more than two isocyanate groups per molecule, through the use of highly-reactive polyether polyols which preferably were provided with primary hydroxy groups by incorporating ethylene oxide units, preferably at the chain end, as well as by the additional use of low-molecular weight polyhydroxy compounds as, for example, glycerol, trimethylolpropane, trimethylolethane, triethanolamine, etc. These measures can be used individually or in combination. In any case, it is important that the crosslinking of the polyurethane foam material takes place so quickly, that, on opening of the foam material, there is no longer a strong collapse tendency and that a uniformity of pore structure is achieved, which is adequate but not excessive, for the application purposes, without having to use a polysiloxane derivative as additive. Foams, which satisfy these characteristics, have such great advantages in regard to physical data and flammability, that they have captured an appreciable portion of the market in recent times. However, such foams, which are usually manufactured as molded foams, exhibit so great a disorder of the foam structure in the boundary or edge zones (large bubbles or contraction cavities), that their practical value is greatly diminished.
Special organosiloxanes have already been proposed as additives for such cold foams. These organosiloxanes are intended to be suitable for eliminating the disorders (See German Offenlegungschrifts Nos. 2,221,811, 2,337,140 and 2,246,400). However, market products, that are based on this principle, still have disadvantages, to wit, poor reproducibility, a relatively high applications concentration and, in some cases, an insufficiently calibrated ratio of regulating effect in the boundary zones of the foam to shrinkage tendency.